The technical field of the invention is that of prefilled and disposable needleless injection devices functioning with a gas generator and used for intradermal, subcutaneous and intramuscular injections of liquid active principle for therapeutic use in human or veterinary medicine.
The active principle is composed of a more or less viscous liquid, a mixture of liquids, or a gel. The active principle can also be a solid dissolved in a suitable solvent for injection or can be a powdered solid in suspension at a certain concentration in a suitable liquid. The particle size of the active principle must then be compatible with the diameter of the conduits so as to avoid blocking them.
In the prior art, needleless injection devices have already been the subject of several patent applications.
Patent application WO 00/46854 relates to a disposable needleless injection device for injecting a variable quantity of liquid active principle. This device more particularly comprises a reservoir of liquid in which a piston is placed for pushing the liquid through an injection system. This device includes a reserve of gas and a device allowing this reserve of gas to be pierced so as to release the gases necessary for pushing the piston present in the reservoir of liquid and thus eject the liquid out of the device. Depending on the nature and/or quantity of liquid active principle to be injected for the particular treatment and depending on the depth of skin penetration desired for said active principle, it is necessary to be able to adapt the quantity of gas to be generated. In the case of a prefilled and needleless injection device ready for use by the user, this choice will have to be made definitively during the process of assembling the device.
In the process of assembling a device of the kind disclosed in patent application WO 00/48654, the gas reserve is positioned, in a first stage, in the body of the device, after which, in a subsequent stage, the reservoir of liquid to be injected is fixed on said body. The compulsory sequence of these two stages is particularly restrictive insofar as it is impossible to easily adapt the quantity of gas present in the gas reserve to the nature and/or quantity of liquid to be injected and to the desired depth of penetration.
The U.S. Pat. No. 4,941,880 discloses a needleless injection device in which a reserve of gas is screwed onto one end of the device independently of the reservoir of liquid. However, the reserve of gas can be fitted onto the device only when the system for piercing the reserve of gas is arranged in the body. This constraint will therefore have to be taken into consideration when assembling such a device.
Moreover, in a device of this kind, it may prove dangerous to fix the reserve of gas onto one end of this device, since said reserve of gas is then easily accessible to the user and may well be damaged.